Light olefins like ethylene, propylene and butylenes are considered as the major building blocks for the production of various petrochemicals. These chemicals are widely used for the production of polyethylene, polypropylene, di-isobutylene, polyisobutylene etc. Conventional steam cracking process remains the major source of light olefins, mainly ethylene and propylene to the petrochemical industry. In the emerging scenario, the demand growth of propylene as petrochemical feedstock is expected to be much higher than that of ethylene. Propylene is the major byproduct from the steam cracking process, which contributes about 70 % of world's propylene demand. About 30% of world's propylene demand is from the conventional Fluid Catalytic Cracking (FCC) units. In recent years, there is a significant gap between the demand and supply of propylene. Consequently, the industry is in the lookout for technology for augmenting production of light olefins. To bridge the gap between the demand and supply of propylene, a new catalytic process is required for production of propylene as the primary product.
Fluid Catalytic Cracking (FCC) process is well known since 1942. The history and the evolution of FCC process at various generations are detailed in the book “Fluid Catalytic Cracking Handbook” by Reza Sadeghbeigi, Gulf publishing company, “Fluid Catalytic Cracking” by Wilson, and various other literatures.
In general, cracking is defined as breaking down of hydrocarbons of higher molecular weight into lower molecular weight hydrocarbons. It can be carried out thermally or catalytically. In fluid catalytic cracking process, the catalyst is a fluidizable fine particle in the size range of 5-150 microns. The steps involved in the conventional FCC process are described below:                i. Hydrocarbon feedstock is preheated to a temperature range of 150-400° C. to enhance the atomization/vaporization of feed;        ii. The preheated feed is mixed with the steam at particular ratio and passed through a nozzle to disperse the feed into fine droplets inside an up-flow riser;        iii. The dispersed feed gets contacted with the hot regenerated catalyst at the bottom of the riser, where the reactions are initiated to take place along the remaining length of the riser;        iv. The mixture of catalyst and products of catalytic cracking is separated by a termination device; further, the entrained catalyst is separated from the product vapor by cyclone separators and transferred to the catalyst bed in the reactor stripper;        v. The entrapped hydrocarbon components are removed from the separated catalyst by stripping using steam;        vi. The coke laden fluidizable catalyst, often referred as spent catalyst, is transferred to a regenerator through spent catalyst standpipe and spent catalyst slide valve;        vii. The deposited coke in the catalyst is burnt in the regenerator using air and the hot regenerated catalyst is transferred to riser through regenerated catalyst standpipe and regenerated catalyst slide valve for the next cycle of operation.        
In this manner, FCC process is termed as a cyclic process where the reaction and regeneration takes place continuously in a riser (reactor) and regenerator respectively. A particular amount of fresh catalyst is added to the circulating inventory in order to maintain the activity of the catalyst while keeping the inventory at constant level.
In the present scenario, as worldwide crudes are becoming heavier, processing of heavy crudes has become important, especially to increase the profit margin. Because of this, it is preferable to maximize the intake of vacuum residue or atmospheric residue in feed to FCC/RFCC unit. However, increase in concentration of heavy ends in FCC unit feed will have several deleterious effects in the known resid FCC units. The associated problems in processing heavy residue in the FCC units are as follows:                i. Excessive coke with the residue produces large amount of excess heat in the regenerator and therefore, the heat balance of the reactor regenerator results in lower conversion.        ii. Higher metal level on the resid leads to significant deactivation of the catalyst and requires incremental catalyst addition to keep the metal level on equilibrium catalyst within acceptable range.                    Crackability of some of the residues, in particular aromatic residues, are not quite good leading to lower conversion.                        iii. Strippability of the heavier unconverted residue inside the catalyst pores is not efficient resulting in higher regenerator temperature and thereby lower conversion.        
The excessive coke in the catalyst generates lot of heat while burning in the regenerator, which limits the catalyst circulation rate to the riser reactor zone, thereby reduces the overall conversion. In order to mitigate this problem catalyst coolers are used conventionally in the resid FCC units, which cools the catalyst indirectly using steam/water as the coolant. These coolers are disclosed in the U.S. Pat. Nos. 2,377,935, 2,386,491, 2,662,050, 2,492,948, and 4,374,750.
U.S. Pat. No. 5,215,650 discloses the indirect cooling of the hot regenerated catalyst via shell and tube heat exchanger type reactor where cracking of light alkanes like ethane, propane and butane takes place and then the cooled catalyst is transferred to the riser reactor.
U.S. Pat. No. 4,840,928 discloses the process of converting lower alkanes to olefins in a third bed, external catalyst cooler in which the excess heat from the regenerator is used directly for thermal cracking of lower alkanes mainly propane with a WHSV of not exceeding 5 hr−1 in the said reactor.
Production of light olefins from feed stocks like VGO is disclosed in the U.S. Pat. Nos. 6,656,346, 4,980,053, 6,210,562, 5,846,402, 6,538,169, 5,326,465, and U.S2006/0108260.
Production of light olefins from naphtha range feed stocks are disclosed in several documents like U.S. Pat. Nos. 4,287,048, 5,232,580, 5,549,813, 6,288,298, 3,082,165, 3,776,838, 5,160,424, 5,318,689, 5,637,207, 5,846,403, 6,113,776, 6,455,750, 6,602,403, 6,867,341, 7,087,155, US2001/042700, US2002/003103, US2003/220530, US2005/070422, US2006/10826, WO2000/18853, WO2002/26628, WO2004/078881, WO2006/098712. Catalytic Cracking of lighter feedstocks like propane, straight run naphtha, olefinic naphtha to produce significant yields of light olefins has its own limitation for commercial realization due to its less coke which affects the heat balance of the unit i.e. the coke produced during the reaction is not sufficient to produce the enough heat which is required for cracking of lighter feeds.
U.S. Pat. No. 7,611,622B2 discloses a dual riser Fluid catalytic cracking (FCC) process with common regenerator involves cracking of first hydrocarbon feed in first riser and cracking of second hydrocarbon feed comprising light hydrocarbons including C3 and/or C4 hydrocarbons, in second riser to form second effluent enriched in light olefins and aromatics. Moreover this invention uses gallium included catalyst to promote aromatics formation.
Chinese patent CN101522866A discloses a dual riser FCC process, wherein first and second hydrocarbon feeds (first hydrocarbon is olefin and the second hydrocarbon feed is paraffinic) are supplied to the respective first and second risers to make an effluent rich in ethylene, propylene and/or aromatics and the respective risers can have different conditions to favor conversion to ethylene and/or propylene.
Some patent literatures, like U.S. Pat. No. 6,113,776, US2002/0003103, U.S. Pat. No. 7,128,827 disclose the concept of dual riser or multiple riser cracking where the portion of the catalyst is used for cracking the lighter hydrocarbons like naphtha range feed stocks and the other portion of catalyst is used in the conventional FCC riser. U.S. Pat. No. 5,846,403 discloses the process in which the naphtha is injected in the same reaction zone but at different elevations of the riser reactor.
None of the cited patents mention about the simultaneous catalytic cracking of lighter feed stocks and heavier feed stocks in different reactors operating in different regimes and conditions to produce significant amount of light olefins and aromatics like benzene toluene, xylene, ethyl benzene etc.
An aim of the present invention is to provide a new catalytic cracking process for simultaneously cracking lighter and heavier hydrocarbon feedstock to produce light olefins and liquid aromatic products.
Another aim is to provide a multiple reaction zone system that enables the production of light olefins and liquid aromatic products both from lighter and heavier hydrocarbon cracking.
Yet another aim of the invention is to provide a catalyst system that can crack both lighter and heavier hydrocarbon under wide range of operating conditions.
A further aim of the present invention is to reduce the sulfur content of the cracked products boiling in the range of C5 to 150° C. from first reaction zone by not less than 60 wt %.
Another aim of the invention is to utilize the excess heat generated in the regenerator due to excess coke burning, which in turn is due to processing of heavier feedstocks in the second reaction zone, effectively in the first reaction zone for cracking of lighter hydrocarbon feedstocks, thereby reducing the temperature of the catalyst entering into the second reaction zone.
Another aim of the invention is to provide a suitable apparatus for carrying out the said new catalytic process.